The present invention relates generally to an improved control means for use in systems which separate gas mixtures by pressure swing molecular sieve absorption techniques, and more specifically to the improved control means as applied to oxygen generating systems for oxygen enrichment and control.
The need for oxygen enrichment and/or oxygen composition control has been well documented in the prior art in diverse areas such as providing proper aircrew breathing mixtures in varying altitudes and providing specially constituted breathing mixtures for individuals undergoing medical treatment. The systems used to supply such special requirements commonly utilize an apparatus which employs absorptive materials designed to absorb and retain particular gas types, such as nitrogen. The most common system process is the pressure swing absorption technique.
A typical prior art pressure swing absorption system apparatus 110 is shown in FIG. 1. Inlet Air Supply 111 is applied to Pressure Regulator 112. Pressure Regulator Output 113, which is of limited pressure variation, is applied to First Input Valve 115 and Second Input Valve 120 as shown. First Absorber Bed Input 116 is supplied through First Input Valve 115. When First Input Valve 115 is open, First Vent Valve 125 is closed. With First Input Valve 115 open, air is routed through First Absorber Bed 140 where absorption of undesired gaseous components occurs because of the characteristics of the absorbing materials used in First Absorber Bed 140. After this processing, the output of First Absorber Bed 140 is routed through First Check Valve 150, which when open connects Outlet Gas Mixture 160 to the output of First Absorber Bed 140. Alternatively, when First Input Valve 115 is closed, First Vent Valve 125 is open which connects the air content of First Absorber Bed 140 to Vent 135 so that undesired trapped gaseous components are discharged from First Absorber Bed 140 to Vent 135. This desorption process is further enhanced by a controlled purge flow through Fixed Cross Flow Orifice 151. During this process First Check Valve 150 is closed. After venting of First Absorber Bed 140, the states of First Input Valve 115, First Vent Valve 125 and First Check Valve 150 are reversed and the absorption process will again occur. The cycle of absorb/vent repeats continuously during system operation.
The second half of the system, composed of Second Input Valve 120, Second Vent Valve 130, Second Absorber Bed 145 and Second Check Valve 155 operates in like manner but concurrently with the first half of the system. Second Absorber Bed Input 121 is supplied through Second Input Valve 120. When First Absorber Bed 140 is providing enriched gas mixture to Outlet Gas Mixture 160, Second Absorber Bed 145 is connected to Vent 135; and when First Absorber Bed 140 is connected to Vent 135 Second Absorber Bed 145 is providing enriched gas mixture to Outlet Gas Mixture 160. First Check Valve 150 and Second Check Valve 155 ensure that only the enriched gas mixture is routed to Outlet Gas Mixture 160 and that the venting process does not affect Outlet Gas Mixture 160.
The typical prior art pressure swing absorption system described above has been utilized as the basis for various improvement patents. U.S. Pat. Nos. 3,948,286 and 4,877,429 present improved valve devices for application in this system. U.S. Pat. No. 4,802,899 presents a way of physically arranging apparatus components to achieve system service and maintenance advantages. U.S. Pat. No. 4,567,909 describes a method of using gas flow control across the absorptive beds as a means of controlling the oxygen concentration of the final product gas. Prior art systems do not address two inherent problems encountered in applying on-board oxygen concentration systems to aircraft, which are operated from air sources of limited capacity and limited pressure, and of the dependence of overall system efficiency on the amount of conditioned air consumed during OBOGS operation which represents a power inefficiency that results in reduced aircraft performance.
The first problem not addressed in the prior art, that of operation from air sources of limited capacity and pressure, manifests itself in aircraft applications by the requirement that an effective OBOGS provide proper operation from 8 to 250 pounds per square inch gauge (PSIG) air inlet pressure, whereas prior art systems exhibit significant performance degradation with air inlet pressures below approximately 20 PSIG. PSIG, as is well known in the art, is the pounds per square inch above atmospheric pressure which is approximately 14.7 at sea level. The second problem not addressed in the prior art is the strong need for efficiency in all aircraft systems, and in particular the need for efficient OBOGS operation at critical points in the aircraft performance envelope. For example, any OBOGS inefficiency represents a loss of available engine power which in turn may manifest itself as inefficient fuel utilization or some other deficiency, such as adverse effects on the cooling or heat exchanger design.
There is thus an unmet need in the art to be able to utilize an OBOGS in airborne applications which is efficient and which will operate from limited air supplies and pressures. Therefore, it would be advantageous in the art to be able to describe a control means for molecular sieve on-board oxygen generators which will provide efficient OBOGS operation from limited air inlet supply and pressure.